: Immunosuppressive medications have made allotransplantation a successful therapy for many diseases. However, immunosuppressive medications are still toxic, often not taken by patients, and beset with concerns about the long-term risk of infection and malignancy. Promising alternatives include strategies that induce donor-specific immune tolerance. One protocol based on costimulation blockade developed in our laboratories induces permanent islet and prolonged skin allograft survival in mice and non-human primates. Concerns remain, however, about the potential of peri- and post-transplantation viral illness to compromise safety and efficacy. In our previous funding period, we documented that host safety and graft durability after peripheral transplantation tolerance inductions are adversely affected by virus infection in a time-dependent manner. The theme of this Program Project is to understand the reciprocal interaction of transplantation tolerance induction and virus infection. The Program participants will identify the mechanisms that determine the safety and durability of transplanted tissues in virus-infected hosts that have been peripherally or centrally tolerized by blockade of CD40-CD154 interaction. The first project will use a new model system based on alloreactive TCR transgenic T cells to study how virus infection abrogates peripheral trans- plantation tolerance in na[unreadable]ve mice, and why pre-existing virus-immunity compromises transplantation tolerance induction. The second project will use a costimulation blockade-based protocol that deletes host alloreactive CD8+ T cells and permits engraftment of allogeneic bone marrow in the complete absence of any myeloablative conditioning. It will test the hypothesis that mechanisms by which virus infection prevents bone marrow cell engraftment will be similar to those that compromise allograft survival in peripheral tolerance protocols. The third project will use newly developed technology that permits identification of allo-specific and virus-specific T cells at the single cell level to examine the nature of virus-induced cross-reactive allo-specific T cells and heterologous immunity in virus-induced allograft rejection. Two Core Facilities focused on Virology and Morphology will facilitate the work of these Projects. If the Project's goals are met, we will understand the mechanisms by which virus infection can compromise both host safety and graft durability as transplantation tolerance enters the "real world" of the clinic.